Existing semiconductor chip handlers transfer chips from a test point to a sequence of sorting locations using a conveyer belt which is driven by a sprocket wheel. The conveyer belt is a web of stainless steel which has a linear sequence of sprocket holes along its edge, each of which is engaged by fixed pins or teeth on a sprocket drive wheel. The drive wheel is mounted to an indexing shaft which is sequentially rotated in a start and stop mode so that chips carried by the belt can be deposited at one or several locations designated for a tested chip. A quantity on the order of a million chips per day must be sequentially advanced along the conveyer belt by the sprocket wheel. A precise, indexed displacement is required to precisely position the tested chips to within 0.0005 inch of the location of each of a plurality of sorting stations. The application requires a precise indexing of the belt's position since cumulative errors can occur when the point of engagement of the pins with the holes in the belt is not precisely registered. When a pin does not accurately engage the hole in the belt, the belt can be lifted off the surface of the wheel tending to stretch the belt, enlarge the hole in the belt, and shorten the operating life of the belt. In addition, the cumulative effects of such pin misregistration can result in misfeeds, missorts, and damaged chips in the conveying operation.
Also, because of misalignment between the pins and holes, imprecise indexing between inspection stations takes place. For example, at one inch spacing intervals between the stations, the one inch dimension will become slightly greater when the belt is lifted by misaligned pins seeking to register with a misaligned opening. The arc resulting from the lifted belt introduces an additional error into the system.
These problems have been partially solved by providing a miniature variable drive pin projection mechanism so that the drive pins extend from the circumferential surface of the drive wheel only during a small portion of the 360 degree rotation of the wheel. Thus, the pins will not tend to misregister when they initially contact the belt surface.
However, the pins must have a 14 degree side taper in order to accommodate an approaching pin's extension into a belt hole and its retraction from a belt hole, in order to maintain a continuous contact between each pin's side surface and the inner surface of the respective sprocket hole in the belt. Since the pins are tapered, there is an optimum difference in the diameter between the top and the bottom of the pin. Thus, the pins must always extend at the same height above the circumferential surface of the wheel, when fully extended.
This raises still another problem since the high velocity in the start/stop operation of the wheel and the long duration of use in the semiconductor testing application, will impose great wear on the base of the pin when actuated by a radial cam surface as has been done in the prior art for other applications. Such wear will cause the height of the drive pin when fully extended, to diminish with time and thus cause the diameter of the tapered pin to be reduced at the point of contact with the sidewall of the sprocket hole in the conveyer belt. Excessive wear requires frequent replacement and long "down" time of the test equipment.
Attempts have been made in the prior art to provide variable projection pin sprocket drive wheels for conveyer belts. U.S. Pat. No. 2,102,651 to Sherman, et al., shows a radial cam for moving the drive pins 22 beyond the periphery of the member 18 through holes in the driven cylinder 10 as the member 18 is rotated. A similar disclosure is made in U.S. Pat. No. 2,842,247 to Euth which shows the use of a radial cam mechanism 31 for projecting pins outwardly beyond the periphery of the drive member into the drive holes of the driven member. U.S. Pat. No. 2,815,672 makes a similar disclosure of a radial cam. U.S. Pat. Nos. 4,022,365; 4,033,492; and 4,136,809 also show a radial cam for driving sprocket pins.
However none of these prior art approaches to variable projection pin sprocket drive wheels solves the problems of vibration and great wear on the base of the pin when actuated by the radial cam surface.
Further, the prior art variable drive pin projection mechanisms are not suitable for the space and size requirements of a miniature indexing mechanism utilized in conveying semiconductor chips at such small and precise dimensions.